Diving training?

This may sound a bit odd, but like my wife said, "it's the internet-give it a shot" so here goes. Our chocolate lab has done dove hunting and some other gamebird retrieving. I am thinking about taking it duck hunting but it likes to dive underwater to get objects. Yes it started in a pool and yes it is a lot of fun to watch and yes he absolutely loves it and goes underwater first to retrieve a floating tennisball haha. I was wondering if there are any tips to help gently ween him off the diving behavior so I can start taking him as a duck dog? My oldest has arthritis and can't handle it like he used to. Don't want either to get hurt. Thanks for any tips and GENTLE-he is a very friendly and sensitive dog.